The use of reinforcing materials for cementitious articles, composites, boards and panels is well known in the cement and construction industries. Generally, cement tends to be strong under compression, but tends to crack and break under tensile, shearing, bending, curing, or shrinkage forces, and must therefore be reinforced. For example, roadways that use asphaltic-type cementitious materials degrade over time, resulting in potholes due to reflective cracking, rutting, and rolling up at traffic lights. Reinforcing materials of various types are used to make such roadways more durable.
Other cementitious articles, such as columns, flat slabs, cementitious boards or precast slabs, tiles, and constant cross-section articles such as pipes, deteriorate over time due to use stress, seismic activity, or rough handling during manufacture or installation of the article. These articles may require a reinforcing material to impart sufficient mechanical properties for their intended use. Specifically, cementitious panels or boards, which are typically used for tile underlayment, exterior insulation and finish systems (EIFS), or precast slabs, may contain a core formed of a cementitious material that is interposed between one or more layers of facing material. The facing materials employed typically possess the features of high strength, high modulus of elasticity, and light weight and contribute flexural, impact, and tensile strength to the cementitious composite board, so that it may be handled before and during installation without cracking.
The technology of choice for reinforcing cementitious articles depends upon the function of the article, but can involve the use of steel, carbon, fiberglass, synthetic thermoplastic staple fibers including olefins, polyesters, amides, polyvinyl alcohol (PVOH), and polyvinyl acetate (PVA). For these applications, the reinforcing materials are incorporated as continuous rods, meshes, fabrics, and/or chopped fibers. Some applications require only chopped fibers to improve toughness, durability, and impact strength, to reduce shrinkage, and to prevent cracking during curing. However, many applications require continuous filaments to provide improved tensile and bending characteristics to the cementitious composite. Because most cementitious articles tend to crack and break at very low tensile elongations, the reinforcing materials must possess a high tensile modulus or alternatively a high tensile strength at low elongation to effectively reinforce the cementitious articles.
The predominant reinforcement materials, chosen for both their performance and relative economics are steel and fiberglass. Unfortunately, these materials suffer from a serious drawback: they tend to corrode or degrade when put into the environment in which they are to act as a reinforcement. Reinforcing materials made from steel are susceptible to corrosion, which can result in a “spalling” effect that causes breakup and deterioration of the cementitious article. Fiberglass, on the other hand, is susceptible to strength loss in the alkaline environment that results when common cements, such as Portland cement, are mixed with water during their curing.
To prevent these problems, specialized coatings are required on these reinforcing materials to allow them to retain their strength during their lifetime. Steel rebar, for instance, is coated with epoxies to prevent corrosion of the steel bar. However, it is very difficult to get a uniform coating on steel in the first place, compounded by the ease of damage to the coating due to rough handling of the steel during the manufacture of the cementitious article. Any imperfection in the coating can result in a portion of the article having substandard performance or a failure (such as a crack or break).
Fiberglass is also coated to protect its strength when used in cementitious composites. For example, cementitious boards typically include a fiberglass fabric reinforcement structure that is coated with a protective poly(vinyl chloride) (PVC) coating. The PVC coating, typically applied as a plastisol, is required to protect the fiberglass yarn and fabric from degradation resulting from exposure to the alkali conditions encountered during the curing of the cementitious article. Any imperfections in the coating (as may occur with conventional coating methods, even when using PVC coatings) result in possible sites for alkali attacks, which may be accelerated if heat is applied to cure the cementitious articles. Since it is difficult to get a uniform protective coating, strength loss for the fiberglass is expected. In fact, the specifications for the glass reinforcement for cementitious boards indicate how much strength loss can be tolerated for a particular fiberglass reinforcement. Therefore, to compensate for potential degradation of the fiberglass that may occur at uncoated, or partially coated, sites, excess fiberglass is often included to ensure a minimum necessary amount of strength over the life of the cement articles.
Typical polypropylene fabrics, though they are intrinsically resistant to the alkaline conditions present in cementitious material and do not corrode, are not of sufficient tensile modulus to be used as a tensile reinforcement for cementitious articles, panels, composites and boards. They are generally only used in staple form to reduce cracking during curing in the conventional art.
Accordingly, there remains a need for improved cementitious articles, composites, panels, or boards that are reinforced by an economical, high modulus fabric that both minimizes or eliminates the need to include a protective coating and that retains the beneficial features of other facing materials. Thus, the present invention relates to cementitious articles that are reinforced through the use of fabrics made from unique high modulus polypropylene yarns, which are by their chemical nature resistant to an alkaline medium.